U.S. patent number 7,178,328 [Application Number 11/017,363] was granted by the patent office on 2007-02-20 for system for controlling the urea supply to scr catalysts.
This patent grant is currently assigned to General Motors Corporation. Invention is credited to Charles E. Solbrig.
United States Patent |
7,178,328 |
Solbrig |
February 20, 2007 |
System for controlling the urea supply to SCR catalysts
Abstract
A reductant dosing control system, for use in a Selective
Catalytic Reduction (SCR) system of a motor vehicle includes an
input receiving a NOx feedback signal from an NOx sensor provided
to the SCR system. A base dosing module calculates a required
quantity of reductant to inject in front of a SCR catalyst of the
SCR system based on the NOx feedback signal. The SCR catalyst has
ammonia storage properties. An output signals a reductant metering
mechanism to periodically or continuously inject excess reductant
based on the required quantity of reductant.
Inventors: |
Solbrig; Charles E. (Ypsilanti,
MI) |
Assignee: |
General Motors Corporation
(Detroit, MI)
|
Family
ID: |
36590728 |
Appl.
No.: |
11/017,363 |
Filed: |
December 20, 2004 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20060130458 A1 |
Jun 22, 2006 |
|
Current U.S.
Class: |
60/286; 60/301;
60/299; 60/287 |
Current CPC
Class: |
F01N
3/106 (20130101); F01N 13/009 (20140601); F01N
3/206 (20130101); B01D 53/90 (20130101); B01D
53/9431 (20130101); F01N 3/103 (20130101); B01D
53/9495 (20130101); F01N 3/208 (20130101); F01N
13/0097 (20140603); F01N 3/035 (20130101); F01N
2610/02 (20130101); F01N 2560/026 (20130101); F01N
2900/08 (20130101); F01N 13/008 (20130101); F01N
2570/14 (20130101); F01N 2610/146 (20130101); Y02A
50/2344 (20180101); Y02T 10/12 (20130101); F01N
2340/02 (20130101); Y02A 50/2325 (20180101); Y02A
50/20 (20180101); Y02T 10/24 (20130101); F01N
2560/06 (20130101) |
Current International
Class: |
F01N
3/00 (20060101); F01N 3/10 (20060101) |
Field of
Search: |
;60/282,286,287,299,301 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Denion; Thomas
Assistant Examiner: Edwards; Loren
Attorney, Agent or Firm: DeVries; Christopher
Claims
What is claimed is:
1. A reductant dosing control system, for use in an SCR system of a
motor vehicle, comprising: an input receiving a NOx feedback signal
from an NOx sensor provided to said SCR system; a base dosing
module calculating a quantity of required reductant to inject in
front of a SCR catalyst of said SCR system based on the NOx
feedback signal, wherein the SCR catalyst has ammonia storage
properties; an intermittent dosing module calculating the quantity
of reductant on an intermittent basis by using a base lookup excess
urea quantity and a base on time and calculating an off time based
on the required quantity of reductant; and an output signaling a
reductant metering mechanism to periodically inject excess
reductant based on the required quantity of reductant, including
signaling the reductant metering mechanism to intermittently inject
reductant according to the base on time, the off time, and the
quantity of reductant by turning a bit on and off according to the
base on time and the off time and multiplying the bit by the
quantity of reductant; wherein a period of injection controlled by
the base on time has a predetermined length, while a rate of
injection controlled by a set point and a duration of no injection
controlled by the off time are variable.
2. The system of claim 1, further comprising an enable logic module
making a determination whether a reductant dosing system of said
SCR system is capable of supplying the quantity of reductant under
present circumstances, and selectively communicating a set point
dosing quantity reflecting the quantity of reductant to the
reductant dosing system based on results of the determination.
3. The system of claim 1, wherein said input receives sensory
signals from engine mass airflow sensors of said motor vehicle and
temperature sensors of said SCR system, the system further
comprising a dosing modification module calculating modifiers based
on the sensory signals to tailor the quantity of reductant to
physical conditions of the engine and catalyst, including
temperature and space velocity, and modifying the quantity of
reductant for engine and catalyst operating conditions according to
the modifiers.
4. The system of claim 1, further comprising an NOx flow rate
determination module converting a PPM signal from the NOx sensor to
a mass per time NOx flow rate signal using air flow rate, fuel flow
rate, and NOx molecular weight, wherein said base dosing module is
adapted to calculate a stoichiometric dosing amount of urea for a
32.5% urea/water solution, including calculating the dosing amount
from the NOx flow rate using a urea to ammonia decomposition
property and molecular weights of Urea and NOx, to correct the
stoichiometric dosing amount for additional dilution with water,
and to filter the amount to reduce noise, thereby determining a set
point dosing quantity.
5. The system of claim 1, further comprising a concentration
calculations module estimating NO/NO2 fraction based on oxidation
catalyst (DOC) properties.
6. The system of claim 1, further comprising a concentration
calculations module calculating NH.sub.3/NOx molar ratios.
7. A reductant dosing control method, for use in an SCR system of a
motor vehicle, comprising: receiving a NOx feedback signal from an
NOx sensor provided to said SCR system; calculating a required
quantity of reductant to inject in front of a SCR catalyst of said
SCR system based on the NOx feedback signal, wherein the SCR
catalyst has ammonia storage properties; and periodically supplying
an excess amount of reductant based on the required quantity of
reductant; calculating the quantity of reductant on an intermittent
basis, including using a base lookup excess urea quantity and a
base on time and calculating an off time based on the quantity of
reductant; and intermittently injecting reductant according to the
base on time, the off time, and the quantity of reductant,
including turning a bit on and off according to the base on time
and the off time and multiplying the bit by the quantity of
reductant, wherein a period of injection controlled by the base on
time has a predetermined length, while a rate of injection
controlled by a set point and a duration of no injection controlled
by the off time are variable.
8. The method of claim 7, further comprising: making a
determination whether a reductant dosing system of said SCR system
is capable of supplying the quantity of reductant under present
circumstances; selectively communicating a set point dosing
quantity reflecting the quantity of reductant to the reductant
dosing system based on results of the determination.
9. The method of claim 7, further comprising: receiving sensory
signals from engine mass airflow sensors of said motor vehicle and
temperature sensors of said SCR system; calculating modifiers based
on the sensory signals to tailor the quantity of reductant to
physical conditions of the engine and catalyst, including
temperature and space velocity; and modifying the quantity of
reductant for engine and catalyst operating conditions according to
the modifiers.
10. The method of claim 7, further comprising: converting a PPM
signal from the NOx sensor to a mass per time NOx flow rate signal
using air flow rate, fuel flow rate, and NOx molecular weight;
calculating a stoichiometric dosing amount of urea for a 32.5%
urea/water solution, including calculating the dosing amount from
the NOx flow rate using a urea to ammonia decomposition property
and molecular weights of Urea and NOx; and correcting the
stoichiometric dosing amount for additional dilution with water,
and filtering the amount to reduce noise, thereby determining a set
point dosing quantity.
11. The method of claim 7, further comprising estimating NO/NO2
fraction based on oxidation catalyst (DOC) properties.
12. The method of claim 7, further comprising calculating
NH.sub.3/NOx molar ratios.
13. A selective catalytic reduction system for use in a motor
vehicle, comprising: a Zeolite based catalyst resident in an
exhaust system receptive of NOx exhaust exiting a diesel engine; a
supply of reductant; a reductant dosing mechanism adapted to inject
reductant into the exhaust system before the catalyst according to
a variable reductant dosing setpoint; an NOx feedback sensor
supplying a NOx feedback signal indicating an amount of NOx exhaust
exiting the diesel engine; and a reductant dosing control system
adapted to calculate a required quantity of reductant to inject in
front of the Zeolite based SCR catalyst based on the NOx feedback
signal, to modify the quantity of reductant based on sensory
signals from engine mass airflow sensors of said diesel engine and
temperature sensors of the enclosure, and to communicate a
reductant set point to said reductant dosing mechanism based on the
required quantity of reductant, wherein said reductant dosing
control system is adapted to control said reductant dosing
mechanism to intermittently inject excess reductant into the
exhaust system before the catalyst for periods of time separated by
durations of no injection, wherein at least one of the periods of
time and the durations are calculated based on the required
quantity of reductant, and wherein said reductant dosing control
system is adapted to compare the required quantity of reductant to
a reductant quantity associated with constant operation of said
dosing mechanism at a minimum setpoint.
14. The system of claim 13, wherein said reductant dosing control
system is adapted to estimate NO/NO2 fraction, and to calculate
NH.sub.3/NOx molar ratios.
15. The system of claim 14, wherein said reductant dosing control
system is adapted to convert a PPM signal from the NOx sensor to a
mass per time NOx flow rate signal using air flow rate, fuel flow
rate, and NOx molecular weight, calculate a stoichiometric dosing
amount of urea for a 32.5% urea/water solution, including
calculating the dosing amount from the NOx flow rate using a urea
to ammonia decomposition property and molecular weights of Urea and
NOx, correct the stoichiometric dosing amount for additional
dilution with water, and filter the amount to reduce noise, thereby
determining a base set point dosing quantity.
16. The system of claim 13, wherein a base on time is employed for
the period, and the set point and the duration are periodically
calculated as a function of quantity of reductant.
Description
FIELD OF THE INVENTION
The present invention generally relates to diesel engine control
systems, and more particularly to intermittent or continuous
reductant supply to Selective Catalytic Reduction (SCR) catalysts
based on feedback from an engine out NOx sensor.
BACKGROUND OF THE INVENTION
Selective Catalytic Reduction (SCR) of NOx using urea as a
reductant is well established for NOx emissions reduction on
stationary sources and mobile applications. In the SCR process, NOx
reacts with a reductant, such as pure anhydrous ammonia, aqueous
ammonia, and/or urea, which is injected into the exhaust gas stream
before a special SCR Catalyst. The SCR approach significantly
reduces diesel NOx.
The SCR process requires precise control of the reductant injection
rate. Insufficient injection may result in unacceptably low NOx
conversion. An injection rate that is too high may release
reductant to the atmosphere. The current dosing control system uses
open loop dosing maps, based on engine speed and load, with
temperature modifiers to lookup the required dosing quantity.
However, the open loop controller logic may not result in optimum
NOx emissions elimination, perhaps due to transient operation with
low levels of emissions. Due in part to minimum practical settings
on reductant dosing mechanisms, it is difficult to precisely supply
reductant at these low levels without slippage of reductant into
the atmosphere.
SUMMARY OF THE INVENTION
A reductant dosing control system, for use in a Selective Catalytic
Reduction (SCR) system of a motor vehicle includes an input
receiving a NOx feedback signal from an NOx sensor provided to the
SCR system. A base dosing module performs a calculation of a
quantity of reductant to inject in front of a SCR catalyst of the
SCR system based on the NOx feedback signal, wherein the SCR
catalyst has NH3 storage properties. An output signals a reductant
metering mechanism to periodically or continuously supply an excess
amount of reductant based on the calculation.
Further areas of applicability of the present invention will become
apparent from the detailed description provided hereinafter. It
should be understood that the detailed description and specific
examples, while indicating the preferred embodiment of the
invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the
detailed description and the accompanying drawings, wherein:
FIG. 1 is a functional block diagram of a diesel engine including a
selective catalytic reduction system according to the present
invention;
FIG. 2 is a block diagram of an SCR system in accordance with the
present invention;
FIG. 3 is a set of graphs demonstrating reductant dosing control in
accordance with the present invention;
FIG. 4 is a block diagram of a reductant dosing control system in
accordance with the present invention;
FIG. 5 is a block diagram of a NOx flow rate determination module
in accordance with the present invention;
FIG. 6 is a block diagram of a base dosing module in accordance
with the present invention;
FIG. 7 is a block diagram of a urea dosing quantity modifier
calculation module in accordance with the present invention;
FIG. 8 is a block diagram of an enable logic module according to
the present invention;
FIG. 9 is a block diagram of an intermittent dosing module in
accordance with the present invention;
FIG. 10 is a block diagram of a final limit application module in
accordance with the present invention;
FIG. 11 is a block diagram of a concentration calculations module
in accordance with the present invention;
FIG. 12 is a block diagram of an efficiency calculations module in
accordance with the present invention;
FIG. 13A is a block diagram of a CAN module in accordance with the
present invention; and
FIG. 13B is a block diagram of a reset integration module in
accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description of the preferred embodiment is merely
exemplary in nature and is in no way intended to limit the
invention, its application, or uses. As used herein, the term
module refers to an application specific integrated circuit (ASIC),
an electronic circuit, a processor (shared, dedicated, or group)
and memory that execute one or more software or firmware programs,
a combinational logic circuit, and/or other suitable components
that provide the described functionality.
By way of overview, FIG. 1 illustrates a selective catalytic
reduction system in accordance with the present invention. Therein,
diesel engine 100 exhausts NOx into exhaust system 102. Electronic
control module 104 receives sensory signals from a plurality of
sensors provided to engine 100 and exhaust system 102. These
sensors include mass air flow sensor 106, engine speed sensor 108,
intake air temperature sensor 110, throttle position sensor 112,
engine out NOx sensor 114, exhaust temperature sensors 116, 118,
and 120, SCR catalyst out NOx sensor 122, and/or Delta pressure
sensors 124 and 126.
It should be readily understood that SCR and Diesel Particulate
Filter (DPF) systems may exist separately or together in the same
vehicle. While filters can be employed to remove particulate matter
from engine exhaust in accordance with DPF, SCR can chemically
alter the chemical structure of gaseous emissions using a catalyst
that retains the noxious emissions as a non-gaseous product of the
chemical reaction. Accordingly, Delta pressure sensors 124 and 126
can be employed in a combined SCR/DPF system according to the
present invention, or left out in an SCR only embodiment of the
present invention.
Electronic control module 104 uses one or more sensor signals,
including a signal from engine out NOx sensor 114, to calculate a
reductant dosing set point and communicate the setpoint, such as a
voltage level, to dosing control module 26 of reductant dosing
system 30. In turn, dosing control module 26 causes reductant
metering unit 32, such as a solenoid actuated valve, to inject
reductant from reductant supply 128 into exhaust system 102 at a
point before an SCR catalyst 130. For example, the set point
voltage opens the valve to a position allowing reductant to pass at
predetermined rate. As the voltage set point changes, the rate of
injection changes accordingly.
According to one aspect of the invention, a range of the reductant
dosing system 30 is extended by overcoming the minimum set point
limitation of the reductant metering unit 32. For example, there is
a minimum opening position of a valve, and therefore a minimum rate
of reductant injection. Accordingly, the present invention includes
intermittently injecting reductant into the exhaust system at a
given rate before the catalyst for periods of time separated by
durations of no injection. Accordingly, the set point is changed to
zero during the durations of no injection. In a preferred
embodiment described below with reference to FIGS. 2 8, the period
of injection has a predetermined length, while the rate of
injection and the duration of no injection are variable. However it
is envisioned that any of these three factors may be fixed, while
at least one of the other factors is varied. It is also envisioned
that all of the factors may be varied. A Zeolite based SCR catalyst
is presently preferred to maintain high conversion efficiency
across the catalyst with intermittent injection. However, it should
be readily understood that other, substantially equivalent
catalysts may be employed that have ammonia storage properties.
FIG. 2 illustrates the presently preferred embodiment of a
Selective Catalytic Reduction (SCR) system for use in an automotive
vehicle in accordance with the present invention. Accordingly,
electronic control module 104 employs a reductant dosing control
system 14 to calculate the set point for reductant dosing system
30. The preferred embodiment employs urea as a reductant. The
reductant dosing control system 14 includes NOx Flow Rate
determination module 16, Base Dosing module 18, Urea Dosing
Quantity Modifier calculation module 20, Intermittent Dosing module
50, Enable Logic module 24, Final Limit application module 52,
Concentration Calculations module 22, Efficiency Calculations
module 54, CAN module 56, and Reset Integration module 58.
Electronic control module 104 employs dosing control system 14 to
affect operation of dosing control module 26. Dosing control module
26 is in communication with electronic control module 104 via CAN
bus 28.
Dosing control module 26 operates reductant dosing system 30 to
inject urea into exhaust system 102. Feedback from real time engine
out NOx sensor 114 is used to calculate a required urea quantity
during a standard (i.e., constant injection) dosing mode and
intermittent dosing mode in accordance with the present
invention.
The dosing control strategy of the presently preferred embodiment
includes three primary steps to calculate and supply the correct
(setpoint) amount of urea reductant to the exhaust system. For
example, a signal is used from real time NOx (feedback) sensor 114
to calculate the correct amount of urea required to inject in front
of a Zeolite based SCR catalyst to maintain optimum performance
(NOx conversion efficiency). Also the operation of an existing
dosing unit is enhanced by extending the low end operation through
a process called intermittent dosing. This process periodically
supplies an excess amount of urea and uses the ammonia storage
properties of a Zeolite based SCR catalyst to maintain high
conversion efficiency across the catalyst when the reductant dosing
system 30 (when used as designed) could not otherwise supply any
urea solution. Further, input is used from other sensors, such as
engine mass air flow, and temperature sensors to calculate
modifiers to tailor the calculated urea quantity to the physical
conditions of the catalyst, such as temperature and space
velocity.
Turning now to FIGS. 4 13, reductant dosing control system 14 (FIG.
4) is described in detail. As illustrated in FIG. 5, in NOx flow
rate determination module 16 (FIG. 5), the NOx sensor signal
supplies the PPM signal and it is converted to a mass per time
signal using air flow rate+fuel flow rate as exhaust flow rate (not
shown but supplied to 0, Mexh), and NOX and exhaust molecular
weights (shown and calculated in concentration calculations module
22 (FIG. 4), MW_NOX, MW_Exh). The main outputs of NOx Flow Rate
determination module 16 are NOX Flow Rate (MNOXF, g/s) and Mass
Exhaust Flow (MEF, g/s).
In base dosing module 18, and as further illustrated in FIG. 6, a
stoichiometric dosing amount of urea solution is calculated from
the NOX Flow Rate (MNOXF) using the urea to ammonia decomposition
properties (2 mol NH3 for 1 mol Urea) and the molecular weights of
urea and NOx; this amount is calculated for a 32.5% urea/water
solution (which is the standard concentration available). The base
setpoint dosing quantity is then corrected for additional dilution
(with water, for dilution and density) and filtered to reduce
noise. The main output of base dosing module 18 is base urea
solution flow rate (QUreaSolnBase, g/hr).
Returning to FIG. 4, in urea dosing quantity modifier calculation
module 20, urea dosing quantity modifiers in the form of
multipliers are calculated for engine and/or catalyst operating
conditions as illustrated in FIG. 7. Two main types of modifiers
are calculated. First, a modifier based on catalyst conditions
(temperature (SCR_TexOpt) and space velocity (SCR_SpVel)) is
determined (UreaCatCondMulti), and second a modifier based on
engine speed and load, and catalyst temperature
(UreaEngineMapMulti) is determined. The final base modifier can be
a function of either or both of these methods (which ever best
suits the situation). Two other modifiers are then included (one
based on engine run time and engine acceleration) before the final
modifiers are applied. There are two final modifiers, one for
standard operating mode and one for intermittent operating mode.
Returning to FIG. 4, the main outputs of urea dosing quantity
modifier calculation module 20 are a standard mode modifier
(UreaFinal_Multi) and an intermittent mode modifier
(UreaFinal_MultiInt). These modifiers are multiplied to the urea
base solution flow rate (QUreaSolnBase, g/hr) in MAIN to get
(QUreaSolnStd) and is supplied to intermittent dosing module 50,
and enable logic module 24.
Turning to FIG. 8, in enable logic module 24, a decision is made to
determine if the required urea quantity can be supplied by the
existing dosing System (due to minimum dosing limitations).
Returning to FIG. 4, if the dosing quantity meets the dosing
system's minimum dosing requirement, (QUreaSolnStd, g/hr) is passed
to final limit application module 52. Otherwise, the intermittent
dosing mode is enabled and the intermittent dosing quantity
(QUreaSolnInt, g/hr) calculated by intermittent dosing module 50 is
passed to final limit application module 52. Returning now to FIG.
8, enable logic module 24 also determines whether urea dosing
should occur at all based on SCR catalyst temperature (SCR_TexOpt,
C). Returning to FIG. 4, the main outputs of enable logic module 24
are a dosing enable flag (DosingEnb) and an intermittent dosing
enable flag (IntDosingEnb).
In intermittent dosing module 50, the intermittent dosing quantity
(QUreaSolnInt) is calculated. Turning now to FIG. 9, the
intermittent calculation conducted in intermittent dosing module 50
uses a base lookup excess urea setpoint quantity from a table
(IntDosingQ) or from a multiplier table (IntDosingQ_Mult) times the
standard quantity (QUreaSolnStd) and a base on time, and calculates
the off time based on the setpoint quantity. A timer algorithm then
turns a bit On-1 and Off-0 according to the calculated on time and
off time, and the bit is multiplied by the excess urea dosing
quantity. Returning to FIG. 4, the main output of intermittent
dosing module 50 is intermittent dosing quantity (QUreaSolnInt,
g/hr).
In final limit application module 52, final limiting of the urea
quantity occurs based on dosing system maximum and minimum flow
information, and the urea setpoint quantity is passed back to MAIN
where (QUreaSolnOutFinal, g/hr) is sent to the dosing system
(through the Hardware Level I/O to the CAN Bus). Calcualtion of
(QUreaSolnOutFinal, g/hr) is detailed in FIG. 10, with final limit
application module 52 limiting the input quantity based on the
minimum and maximum dosing system capabilities.
Returning to FIG. 4, other auxiliary modules exist to support the
algorithms and testing, including concentration calculations module
22, efficiency calculations module 54, and reset integration module
58. Turning to FIG. 11, for example, in concentration calculations
module 22, NO/NO2 fractions and instantaneous and cumulative
NH3/NOx Molar Ratios are calculated. Also, turning to FIG. 12,
efficiency calculations module 54 calculates instantaneous and
cumulative NOx conversion efficiencies from pre and post SCR
catalyst NOx sensors. Turning next to FIG. 13A, CAN module 56
further acquires CAN bus information from the dosing system and
scales the acquired information. Yet further, FIG. 13B illustrates
operation of reset integration module 58.
Results of SCR reductant dosing control according to the present
invention are graphed in FIGS. 2 and 3. For example, dosing/no
dosing line 38 delineates the FTP75 operating range 40. This range
is extended as at 36 by use of intermittent dosing when the
required reductant quantity falls below a minimum value that can be
provided by constant dosing. Also, NOx sensor dosing control is
graphed at 42, where urea quantity scales with the engine out NOx.
Further, intermittent and constant dosing are graphed together at
44, with a switch occurring between modes according to the upper
curve. It should be readily understood that intermittent dosing may
be used exclusively in some embodiments, but that durations of no
injection may reduce to zero at times when heavy reductant flow is
required.
The NOx feedback control according to the present invention allows
the urea dosing system to supply the correct amount of urea under
all engine and catalyst operating conditions. Also, unusual engine
operating conditions can be automatically accommodated, such as EGR
malfunction and particulate filter regeneration. As a result, tail
pipe ammonia can be minimized under most circumstances. Further,
the dosing quantity self-adapts to the engine calibration, which
facilitates the engine calibration process.
The intermittent dosing strategy also extends the capability of the
urea dosing system to maintain NH.sub.3/NOx ratio during low NOx
conditions by exploiting Zeolite catalyst NH.sub.3 storage
properties. It also allows the use of a higher concentration
solution to cover both the Low Speed light Load (Low NOx), and the
High Speed High Load (High NOx) conditions.
It is envisioned that additional algorithms may be added to allow
the use of real time converter efficiency calculations or for
On-Board Diagnostics (OBD) functionality (using 2 NOx sensors) and
historical operational information to modify the set point dosing
quantity from long term learned system behavior. It is also
envisioned that the system and methods of the presently preferred
embodiment may be modified to accommodate other reductants, such as
pure anhydrous ammonia, aqueous ammonia, or any form of ammonia
capable of being precisely metered. It should be readily understood
that urea changes into NH.sub.3 through decomposition reactions in
the exhaust system. However, there are 2 moles of NH.sub.3
available for every mole of urea, instead of just 1 for pure
ammonia. Thus, the calculation changes slightly depending on
whether there is NH3 or Urea and the concentration of the
substance.
Those skilled in the art can now appreciate from the foregoing
description that the broad teachings of the current invention can
be implemented in a variety of forms. Therefore, while this
invention has been described in connection with particular examples
thereof, the true scope of the invention should not be so limited
since other modifications will become apparent to the skilled
practitioner upon a study of the drawings, the specification and
the following claims.
* * * * *